Improving Tensile Properties of Dissimilar TLP Bonding Joints of IN718 Nickel-Based Superalloy/316LN Austenitic Steel by Long-Term Post-bonded Homogenization Treatment

The presence of boride precipitates has a detrimental impact on the mechanical properties of TLP bonding joints. This study investigated the effectiveness of the long-term post-bonded homogenization treatment (PBHT) in mitigating the adverse effects of boride precipitates and enhancing the overall performance of TLP joints formed with IN718/316LN. Within the diffusion affected zone on the side of 316LN (referred to as DAZ-316LN), the low concentrations of Cr, Mo, and Nb in boride precipitates, coupled with a significant lattice mismatch at the boundary between boride precipitates and the matrix, facilitate the dissolution of boride precipitates during the PBHT process. This contributes to the improvement of the metallurgical bonding strength at the interface between the isothermal solidification zone (ISZ) and DAZ-316LN (referred to as the ISZ/DAZ-316LN interface), consequently shifting the location of tensile fracture from the ISZ/DAZ-316LN interface towards ISZ. Moreover, the PBHT process induces a breakdown of needle-like or continuously distributed borides along grain boundaries, transforming them into small block or short acicular boride precipitates that are uniformly distributed in the joint. This optimization process not only enhances the precipitation-strengthening effect but also alleviates stress concentration, resulting in an overall improvement in the joint’s strength and ductility.